Gylfi Sigurdsson’s smart finish saw Swansea City claim a surprise 2-1 win at Manchester United as the Louis van Gaal era started in defeat.

Sigurdsson bagged the winner 18 minutes from time on Saturday as Van Gaal – tasked with improving the fortunes at Old Trafford after David Moyes’ disastrous spell in charge last term – endured a day he will want to forget in his opening Premier League fixture.

The Dutchman’s 3-4-1-2 system did not appear to suit United despite being used in pre-season and they found themselves behind when Ki Sung-yueng scored from 20 yards just before the half-hour mark.

Van Gaal, known for his tinkering, swapped to 4-4-1-1 in the second half and it appeared to be the tonic for United’s response, led by new captain Wayne Rooney, who levelled with a clever overhead kick eight minutes after the break.

Rooney saw an outstanding free-kick crash against the post with 24 minutes left and Old Trafford was then stunned when Sigurdsson – Swansea’s main signing of the close-season – finished well after Wayne Routledge’s cut-back.

The result is sure to put early pressure on Van Gaal and is likely to see further spending from United in the transfer window, as they look to recover from last term’s seventh-place finish.

The victory was Swansea’s second at Old Trafford in 2014 – and second ever – as the Welsh club also won a third-round FA Cup tie by the same scoreline in January.